((end_time - moment()) / (1000 * 60)) will give you a value in minutes (1000 milliseconds in a second, 60 seconds in a minute).

You said self.autoextend_increment is already in minutes, so no additional arithmetic is needed. If you really want to compare in seconds, you can: ((end_time - moment()) / 1000) <= self.autoextend_increment * 60

You could have also written your conversion as (end_time - moment()) / 6000 but I broke down the figures for demonstration purposes.

Email codedump link for How to use moment.js to subtract two times and compare it to a number of minutes